Signal collecting system for radio receivers



Jmw W, AWL W. A. sm-iAPm SIGNAL COLLECTING SYSTEM FOR RADIO RECEIVERS Filed Feb. 19, 1940 2 Sheets-Sheet l a W W resomn cwcat neaoncuzf a6 a f/W 5/ near Z/ae Law f negwmag and 0/ the.

mng r INVENTOR .JCHflPE/E ATTORNEY .Wmme 1?, 11941. w SCHAPER 2,2455%51 SIGNAL COLLECTING SYSTEM FOR RADIO RECEIVERS Fi led Feb. 19, 1940 2 Sheets-Sheet 2 INVENTOR W/LL/HMAJc/M'PEE BMQM ATTORNEY Patented June 17, 1941 SIGNAL COLLECTING SYSTEM FOR RADIO RECEIVERS William A. Schaper, Cicero, Ill., assignor to Johnson Laboratories, Inc., Chicago, 111., a corporation of Illinois Application February 19, 1940, Serial No. 319,672

Claims.

This invention relates to high-frequency circuits, such as those employed in radio receiving systems. More particularly, the invention relates to the portion of such systems which constitutes means for collecting the high-frequency signals radiated from relatively distant transmitting stations. This invention incorporates an improved signal-collecting means.

My invention takes advantage of the unique properties of systems which are tuned over a range of frequencies by inductance variation, and makes it possible to employ such systems in a new and highly advantageous manner. One such system is disclosed by Polydoroif in United States Patent No. 1,940,228, in which a resonant circuit having an inductance coil and capacitor is adjusted over a range of frequencies by movement of a compressed comminuted core relatively to the inductance coil. This method of tuning is called permeability tuning. An improved form of such a system is disclosed in my United States Patent No. 2,051,012. Both Polydorofis original system and my improved system readily cover an adequate range of frequencies and may easily be ganged to provide multiple unit systems.

In radio receivers, and more particularly those intended for the reception of broadcasting, it is important that the signal-to-noise ratio at the input to the first vacuum tube be as high as possible. This ratio may be increased either by increasing the amount of signal pick-up or by decreasing the noise pick-up, or by both simultaneously. The signal pick-up may be improved by increasing the effective dimensions of the signal-collecting means or by tuning the circuit including the signal-collecting means to resonance with the desired signal or both.

One form of collector circuit which includes the signal-collecting means and which may be conveniently tuned to resonance with the incoming signal is that employing an exposed inductive element, commonly called a loop. In such arrangements, both terminals of the exposed inductive element are connected to the remaining elements to form a resonant circuit which is closed through the conductor of the exposed inductive element.

It has been common to tune such closed collector circuits by capacitance variation. In the present application, and in my copending application, Serial No. 319,671 filed February 19, 1940, however, I describe closed collector circuits which are tuned by inductance variation, and preferably by employing an additional unexposed inductor made variable by means of a ferromagnetic element movable relatively to the winding thereof. In my copending application just referred to, it is explained that this departure from conventional capacitance tuning produces marked advantages because of the control of circuit performance over the tunable frequency range which ferro-magnetic inductance variation provides in spite of the necessarily reduced inductance values which must be employed in the exposed inductive elements. These advantages, as well as additional advantages later to be described, are secured in arrangements according to the present application.

In my copending application above referred to, I describe a series-tuned collector circuit including a single exposed inductive element which may be a loop, an unexposed variable inductive element and a capacitance. The arrangement according to the present invention, however, employs plural exposed inductive elements forming one fixed-tuned circuit and one variably-tuned circuit including a variable unexposed inductive element and a capacitance, and I thus secure plural characteristics which, when properly employed in accordance with the instructions herein given, provide a highly efiicient and compact collector system having excellent uniformity over the frequency range as to both resonant gain and selectivity.

An object of my invention is to provide an improved signal-collecting means for radio receivers.

Another object of my invention is to provide a good signal-to-noise ratio with a relatively small and compact collector.

An additional object is to provide a signalcolleoting means which may be successfully employed in the most compact forms of radio receivers, and which may be placed in close proximity to the receiver chassis without serious detriment.

Still another object of the invention is to provide a signal-collecting circuit whose performance characteristics may be readily controlled with respect to variation with frequency.

It is also an object of the invention to provide a signal-collecting circuit which may be tuned by inductance variation, for example by means of a movable ferromagnetic core, and in which the advantages of this method of tuning may be realized.

These and other objects are realized in accordance with my invention in a manner which will be more readily understood by reference to the accompanying drawings, in which:

Fig. 1 is a schematic diagram of the arrangement according to the invention;

Fig. 2 is an equivalent circuit diagram corresponding to Fig. 1;

Figs, 3 and 4 are vector diagrams indicating the relations existing in the circuit of Fig, 1 and equivalently in the circuit of Fig. 2; and

Fig. 5 shows a form of compound loop suitable for use in certain embodiments of the invention.

The arrangement according to Fig. 1 comprises a first exposed inductive element I in series with a capacitance 3, a second exposed inductive element 2, an unexposed inductive element 4 and a capacitance 6, connected as shown. A ferromagnetic element 5 is arranged to be movable relatively to inductive element 4 to vary its inductance. The resultant signal voltage developed across capacitance 6 is applied to the control grid I of vacuum tube 8, which may be the first vacuum tube in a radio receiver.

Exposed inductive elements I and 2 preferably take the form of loops, and may be of any physical size consistent with the size of the receiver or other apparatus with which they are to be used. Because of the very high resonant gain secured in accordance with the invention, however, loops I and 2 may be made quite small without detriment, and I shall later describe a construction suitable in the case in which the loops are to be incorporated in a compact receiver.

The arrangement of Fig. 1 includes a variablytuned resonant circuit having a compound inductive leg comprising inductors 2 and 4 in series with capacitor 6, and a fixed-tuned resonant circuit comprising inductors I and 2 in series with capacitor 3. Inductors I and 2 are so positioned that there is a small amount of inductive coupling between them. Inductors I and 2 are exposed to the signals and a signal voltage is, therefore, generated in each of them. The inductance of inductor 4 is varied by ferromagnetic element 5 to tune the system for maximum response to any desired signal.

It will be apparent that since the variably tuned circuit includes inductances 2 and i, the Variable inductor i5 must provide greater inductance variation than would be required if the inductance 2 were absent, in order to tune the system over the required frequency range. It may be shown that, if L is the eifective external series inductance, p is the ratio of the highest to the lowest frequency in the required tuning range, and La is the minimum inductance of the variable inductor l5, then the required effective permeability p of ferromagnetic element is Referring now to Fig. 2, R1 and X1 represent the resistance and inductance respectively of inductor I of Fig. 1, R2 and X2 represent the resistance and inductance respectively of inductor 2 of Fig, 1 and R4 and X4 represent the resistance and inductance respectively of variable inductor 4-5 of Fig. 1, it being remembered that X; is variable to tune the system over a range of frequencies, that R4 is also controllably variable by suitable design of ferromagnetic element 5 to control the total effective resistance of the system at all frequencies within the range, and that R1 and R2 also vary with frequency, but with suitable construction of inductors I and 2 may be relatively very small and have relatively minor variation. The mutual inductance between inductors I and 2 of Fig. 1, corresponding to reactances X1 and X2, in Fig. 2, is indicated in Fig. 2 by the symbol X9 and the arrows. Capacitive reactance Xe in Fig. 2 corresponds to capacitor 3 in Fig. 1. Capacitive reactance X6 in Fig. 2 corresponds to capacitor 6 in Fig. 1. The voltages generated in inductors I and 2 of Fig. 1 by any signal are indicated in Fig. 2 by the symbols E1 and E2 respectively.

The highly advantageous performance of the system depends upon the fact that it possesses two principal characteristics, one due to reactances X2, X4 and X6, and the other due to reactances X1, X2 and X3 in series resonance. By appropriate choice of constants, including the inductance-to-resistance ratio of variable inductor 45 as it is adjusted to produce resonance over the frequency range, these two characteristics may be caused to produce a desired over-all characteristic for the complete circuit, with high gain throughout the frequency range, as will now be explained.

As is well known, the current generated in an exposed conductor by any radio signal is directly proportional to the frequency of the signal,

whether the exposed conductor be of the openended antenna type or the closed loop type. Thus if the voltage delivered at the grid of the first vacuum tube of the receiver is to be directly proportional to the strength of the signal, as it preferably should be, the resonant gain of the collector circuit should be inversely proportional to the frequency of the signal. Such a characteristic cannot be secured in a circuit tuned by capacitance variation, because in such systems there is no control of the high-frequency resistance of the circuit, and the variation in the resistance of the inductive elements themselves is entirely inadequate to produce the desired result, as experience has shown. Nor is it possible to produce adequate control of the circuit resistance entirely by the action of the ferromagnetic elements in a circuit tuned solely by inductance variation, since these elements inevitably increase the high-frequency resistance as they are inserted into an inductive winding to tune the circuit to the lower frequencies. It is therefore essential that additional means for suitably varying the gain characteristics of the circuit be provided, as contemplated in the present invention.

Again referring to Figs. 1 and 2, the exposed inductive elements I and 2 are fixedly positioned so as to produce a small inductive coupling between them. The resulting coupling reactance, indicated by X9 in the equivalent circuit diagram of Fig, 2, varies directly with frequency. This variation, properly correlated with the control of circuit resistance provided by variable inductor 35, produces the desired relation of voltage at the grid 1 of the tube 8 proportional to the strength but independent of the frequency of the selected signal. It will be apparent that depending upon how-inductors I and 2 are connected, the coupling reactance X9 will be additive or subtractive. Either relationship may be employed without sacrificing the advantage of grid voltage proportional to signal strength regardless of frequency, but it will be found that the subtractive relation is usually preferable because of relatively higher gain at all frequencies.

The resonant frequency of the fixed-tuned circuit comprising exposed inductors I and 2, and capacitor 3 will be that at which the inductive reactance, X1 plus X2 plus or minus twice the mutual reactance X9, is equal to the capacitive reactance X3. The voltages E1 and E2 will be proportional to the number of turns in inductors I and 2 respectively. In preferred embodiments, exposed inductor I is arranged to provide increased signal voltage at the lower end of the frequency range, and to this end the fixed-tuned circuit just described is made resonant at a relatively low frequency. Since the permissible inductance of inductor 2 is limited by the available inductance variation of variable inductor 4-5, and since it is desirable to generate a relatively large signal voltage in inductor I and to employ relative small inductive coupling between inductors I and 2, inductor I preferably has a relatively high inductance value and capacitor 3 is made relatively small. For particular purposes, however, the fixed-tuned circuit may be made resonant at any desired frequency by appropriate choice of inductor I and capacitor 3.

Assuming that inductor I and capacitor 3 are such that the fixed-tuned circuit is resonant at or near the lower frequency end of the range, they will have a relatively very small effect upon the resonant frequency of the variably-tuned resonant circuit comprising inductors 2 and 4-5 and capacitor 6 at or near the high frequency end of the band. Although the complete characteristic of the system is complex, as will be indicated by the formulae soon to be given, it may be stated broadly, therefore, that the variably-tuned circuit may be given a desirable performance characteristic by appropriate choice of ferromagnetic element 5, and the fixed-tuned circuit may be given an inverse frequency-voltage characteristic to compensate for the normally rising frequency-voltage characteristic of the variably-tuned circuit.

Figs. 3 and 4 show the approximate vector relationships of the several voltages and currents for the subtractive and additive relationships of inductors I and 2 in the circuit of Fig. 1 or correspondingly of reactances X1 and X2 of Fig. 2, it being understood that these vector diagrams are purely illustrative, and that the magnitudes and phases of each of the vectors may vary considerably from those shown, in physical embodiments of the invention. It will be seen that the resultant signal-frequency voltage E between grid I and ground, which is the voltage drop across reactance X6, will normally be considerably higher for the subtractive relationship of Fig. 3 than for the additive relationship of Fig. 4, and this is found to be true in practical embodiments.

From the equation between the sum of the voltages E1 and E2 generated by any signal in exposed inductors l and 2 respectively, and the sum of the voltage drops in the circuit, it may be shown that the reactance and effective highfrequency resistance of the circuit are, respectively The quality factor of the circuit is in which H=(R1l--R2) +D In practical embodiments of the invention, certain portions of the above equations become negligibly small at either the high-frequency or the low-frequency end of the tuning range or both, so that, to a very good approximation, at the high-frequency end of the range It may be shown that, as the vector diagrams of Figs. 3 and 4 indicate, the coupling between inductors I and 2 may be chosen at a value which will make the quality factor of the circuitas given by Equation 3 a maximum. The required value is relatively small and is most conveniently determined experimentally in any particular embodiment. An indication of its proper magnitude will be gained from the detailed description of a successful embodiment of the invention to be given later.

It will be noted that variable reactance X4, which is the sole means of tuning the circuit over the frequency range, occurs only once and only in the numerator of Equation 3. This reactance is inversely proportional to the frequency, the remaining inductive reactances being directly proportional to frequency. Similarly, it will be noted that the effective resistance R4 of the variable inductor 45 occurs only once and only in the denominator of Equation 3. Whatever the variation with frequency of the remaining resistances may he, therefore, it is apparent that by appropriate choice of ferromagnetic element 5 to produce a desired variation with frequency in resistance R4, the quality factor Q, as given by Equation 3, may be held substantially constant or may be given any desired variation with frequency.

At resonance the effective inductive and capacitive reactances cancel, the resulting current through capacitance 6 being determined only by the effective resistance of the system. To a very close approximation this current will be, at the high-frequency end of the range and at the low-frequency end of the range It will be apparent from Equations 8 and9 that (Xz-l-Xo) should be made as large as possi-ble, remembering that it may not be greater than the value which will permit the variable inductor 45 to tune the circuit over the required frequency range.

The selectivity of the system resides in the variably-tunable circuit X2X4Xs, and, as is well known, is proportional to the ratio of inductance to resistance, L/R, in that circuit. Asis also well known, this ratio may be kept substantially constant by appropriate design of ferromagnetic element 5 and inductor 4, so as to secure constant selectivity. If this is done, however, the resonant gain of circuit X2X4X6 will not be constant, but will be greater at the higher frequencies. The gain characteristic of fixed-tuned resonant circuit XIXZX3 may besuch as to compensate for this additional cause of variation, preferably in such a way as to provide a voltage at the grid 1 directly proportional to the strength but independent of the frequency of the selected signal.

By way of illustrative example of a practical embodiment of the invention having the preferred characteristics described above, the following circuit constants and construction data are given, it being understood that they are not to be taken as in any way limiting the scope of the invention, which resides in the novel circuit arrangement as described in the appended claims, rather than in the particular constants or construction employed.

Exposed inductors l and 2 may be close-wound single-layer solenoi-dal loops on six inch square forms, and having windings of plain enamelled wire as follows:

Inductor 1 2 Turns 68 12 Wire number 34 28 Inductance l9 h. 82,1111.

Inductor may comprise a progressive universal winding of 7 #14 single silk enamelled litz wire 1% long on an insulating tube of 0.205" inside diameter and 0.222 outside diameter, and having an inductance of 200 ,uh., a Q of 110 at 600 kc., and a Q of 80 at 1560 kc.

Ferromagnetic element 5 may comprise a compressed comminuted core of hydrogen-reduced powdered iron that has been sifted through a screen having 400 meshes to the inch. For use with the above described inductor, it may be 0.200 in diameter and 1% long, being preferably hot-molded at 180 F. with 0.5% particle insulation and 3% of powdered Bakelite binder, and cured at 290 F. for 3 hours. Such a core will have an effective permeability of about 11.5, so that the maximum inductance of variable inductor t-5 will be of the same order as that of inductor i. For use with the above described inductors, capacitors 3 and 6 may be of ,lL/Lf. and 3'7 LL/.Lf. capacitance respectively. With these constants the fixed-tuned circuit will be resonant at approximately 600 kc. and the variably-tunable circuit will cover the range from 540 kc. to 1560 kc. The gain ratio of the system at the highfrequency end of the range to that at the lowfrequency end of the range, will be of the order r of 0.25 to 0.6.

Fig. 5 shows a desirable construction for inductors i and 2 when they take the form of small loops such as those just described. The forms it, may be identical and may suitably be constructed of hard wood, or, if desired, a molded form of insulating material may be used. The required small degree of coupling is secured by placing the loops substantially in the same plane, as shown, with their adjacent edges approximately in" apart, the subtractive relation of the mutual inductance being secured by arranging the winding directions as indicated by the arrows, and by connecting the terminals :1, b and c as indicated by the letters a, band c in Fig. 1.

When used in connection with a radio receiver of the superhetero-dyne type having an intermediate frequency of the order of 460 kc., my improved collector circuit provides exceptionally high discrimination against signals of the socalled image frequency. In the illustrative embodiment above described, for example, the image ratio will be of the order of 50 at the highfrequency end of the range and of the order of 4000 at the low-frequency end of the range.

This is due to the fact that while inductor l provides a very considerable portion of the total pick-up at the lower frequencies Where the image response is most troublesome, the fixed-tuned circuit including inductor l is very far from resonance with the image signals and thus very effectively excludes them. This high image ratio, therefore, is an inherent and highly advantageous attribute of my invention.

Having thus described my invention, what I claim is:

1. A signal-collecting system for use in radio receivers and the like including a fixed-tuned resonant circuit comprising two exposed inductive elements and a capacitor in series, a variablytuned resonant circuit comprising one of said inductive elements, a second capacitor and an unexposed inductive winding in series, and means for tuning said tunable circuit to resonance with a desired signal solely by variation of the inductance of said winding.

2. A signal-collecting system for use in radio receivers and the like including a fixed-tuned resonant circuit comprising two exposed inductive elements and a capacitor in series, a variablytuned resonant circuit comprising one of said inductive elements, a second capacitor and an unexposed inductive winding in series, and means including a ferromagnetic element movable relatively to said winding for tuning said tunable circuit to resonance with a desired signal solely by variation of the inductance of said winding.

3. A signal-collecting system for use in radio receivers tunable over a range of frequencies, including a fixed-tuned resonant circuit comprising two exposed inductive elements and a capacitor in series and resonant near the low-frequency end of said range, a variably-tuned resonant circuit comprising one of said inductive elements, a second capacitor and an unexposed inductive winding in series, and means including a ferromagnetic element movable relatively to said winding for tuning said tunable circuit to resonance with any desired signal within said range solely by variation of the inductance of said winding, said two exposed inductive elements being slightly coupled electro-magnetically.

4. A signal-collecting system for use in radio receivers and the like including a fixed-tuned series resonant circuit comprising a capacitor and two exposed inductive elements of materially different inductance values, a variably-tuned resonant circuit comprising the smaller of said inductive elements, a second capacitor and an unexposed inductive winding in series, and means for tuning said tunable circuit to resonance with a desired signal solely by variation of the inductance of said winding.

5. A signal-collecting system for use in radio receivers tunable over a range of frequencies, including a fixed-tuned series resonant circuit comprising a capacitor and two exposed inductive elements of materially different inductance values, said circuit being resonant near the lowr frequency end of the range, a variably-tuned resonant circuit comprising the smaller of said inductive elements, a second capacitor and an unexposed inductive Winding in series, and means including a ferromagnetic element movable relatively to said winding for tuning said tunable circuit to resonance with any desired signal within said range solely by variation of the inductance of said winding, said two exposed inductive elemelnts being slightly coupled electro-magnetL ca y.

6. A signal-collecting system for use in radio receivers tuna-ble over a range of cfrequencies including a tunable series resonant circuit comprising an exposed inductive element, a capacitor and a variable inductor, and means including a second exposed inductive element and a second capacitor forming with said first-mentioned exposed inductive element a fixed-tuned series resonant circuit for producing across said firstmentioned capacitor a signal voltage substantially proportional to the strength of a selected signal and substantially independent of the frequency of said signal.

7. A signal-collecting system for use in radio receivers tunable over a range of frequencies including a tunable series resonant circuit comprising an exposed inductive element, a capacitor and a variable inductor, and means including a second exposed inductive element and a second capacitor forming with said first-mentioned exposed inductive element a fixed-tuned series resonant circuit resonant near the lowfrequency and of said range for producing across said first-mentioned capacitor a signal voltage substantially proportional to the strength of a selected signal and substantially independent 01' the frequency of said signal.

8. A signal-collecting system for use in radio receivers tunable over a range of frequencies including a tunable series resonant circuit comprising an exposed inductive element, a capacitor and a variable inductor having a winding and a term-magnetic element movable relatively to said winding, and means including a second exposed inductive element and a second capacitor forming with said first-mentioned exposed inductive element a fixed-tuned series resonant circuit for producing across said first-mentioned capacitor a signal voltage substantially proportional to the strength of a selected signal and substantially independent of the frequency 01' said signal.

9. A signal-collecting system for use in radio receivers tunable over a. range of frequencies including a tunable series resonant circuit comprising an exposed inductive element, a capacitor and a variable inductor having a winding and a term-magnetic element movable relatively to said winding, and means including a second exposed inductive element and a second capacitor forming with said first-mentioned exposed inductive element a fixed-tuned series resonant circuit resonant near the low-frequency end of said range for producing across said first-mentioned capacitor a signal voltage substantially proportional to the strength of a selected signal and substantially independent of the frequency of said signal.

10. A signal-collecting system for use in radio receivers tunable over a range of frequencies including a tunable series resonant circuit comprising an exposed inductive element, a capacitor and a variable inductor having a winding and a term-magnetic element movable relatively to said winding and having a maximum inductance value of the order of twenty times the inductance of said exposed inductive element, a second capacitor, and means including a second exposed inductive element having an inductance value of the same order as the maximum inductance value of said variable inductor, and forming with said second capacitor and said firstmentioned exposed inductive element a fixedtuned series resonant circuit resonant near the low-frequency end of said range for producing across said first-mentioned capacitor a signal voltage substantially proportional to the strength of a selected signal and substantially independent of the frequency of said signal.

WILLIAM A. SCI-IAPER.

v CERTIFICATEOF CORRECTION Patent 110., -2,2l 6;585. June 17, 19m. WILLIAM'A. SCHAPER.

it is hereoy certified that error appears in the. printed specifica'tion of theabove numbered patent requiri ng correction as follows: Page .l second colku mn, linei, gfter provid es" inserta comma; page 5, first columri,

line 68, insert the equation number "(5)" ap'the righd-hand end of the line; same pegs, second column, line 51, in the left-hand portion of the equation, for "ll-' read --I page 5', first column, line 25, claim 7,

v for "and" read "endand "chat thesaid Letters Patent should be read with this correction tt1erei.1l1:hza1t the same may conform to the record of the case in the Patent Office. v

Signed and sealed this 19th dayof August, A. D. l9l il.

} Henry Van Arsdale, (Seal) Acting Commissiorrer of Patents; 

